Preventing feed-back in amplifier tubes having common plate supply



Feb 16, 1932.

J. R. HYNEMAN PREVENTING FEED BACK IN AMPLIFIER TUBES HAVING COMMON PLATE SUPPLY Filed April 14, 1931 I 7 5 A E ower "JP j a @2223 g 4 p H a, I l I; c' 5 E g gmmtoc (1. 1?. 175/220220/2 Patented Feb. 16, 1932 UNITED STATES PATENT OFFICE JOHN R. HYNEMAN, F GHATHAM, NEW JERSEY, .ASSIGNLOB TO THE WESTERN UNION TELEGRAIH COMPANY, OF NEW YORK,

PREVENTING FEED-BACK IN AMPLIFIER Application filed April 14,

This invention relates toa signaling system, and in particular :to a signaling system employing a plurality of vacuum tube relays connected in cascade and provided with a common source of space current.

An object of the invention is to devise a multistage vacuum tube relay or amplifier system in which the vacuum tubes of the various stages are supplied withspace current from a common source and in which means is provided for preventingthe variations in current in one stage from aifectinga preceding stage through the mutual impedance of the common supply source.

My invention is illustrated in the accompanying drawings in which:

Figure 1 is a circuit diagram showing a common :type of cascade vacuum tube amplifier, and t Figure 2 shows the circuit arrangement of the same type of amplifier showninFigure 1 having my invention embodied therein.

Referring to Figure 1, the numerals 1, 2, 3 and l indicate vacuum tube relays employed in the four stages of the amplifier. I prefer to employ tubes having heater type cathodes for the first three tubes of the amplifiergsince these tubes can use a common source of heater currentwhile retaining separate cathode connections. Tube 1 is a radio frequency am plifying stage, tube 2 serves as a detector,

while the third and fourth stages amplify the detected low frequency signals, the last tube being of a size and type to supply the requithe receiving device. Space currentfor all of the relays is supplied from an alternating current source 5 through rectifier 6, a filter 7 and a potentiometer or voltage divider resistance 8 connected across the terminals of the filter 7. The cathode heaters are shown as supplied with alternating current from low voltage secondary windings of the power transformer 5, but they may be supplied. from any suitable source. The cathodes of tubes in the various stages are grounded, and an intermediate point 9 of resistanceS is also grounded. Relays *1, 2 and 3 are supplied withplatecurrent from a tap 10 onresistance 8,:andrclay 4 supplied with plate current in other ty es as well. (Dnecommonmethod site amount of power for the operation of N. Y., A CORPORATION 01? nnwyonx runes HAVING common rmrrn .SUPIIiY 1931. Serial No. 530,083.

of maximum voltage from the positive terminal of the potentiometer. The portion of the potentiometer resistance 8 lyin to the left of the grounded point, 9 is uti ized for providing proper biasing potentials for the grids of the various stages. Relays 1, 2 and?) are each provided witha plate circuit filter comprising series inductances l 25,, hand shunt condensers 1 2 3 p 3 l 1 The last tube 4 shown in Figure 1 iscommonly calledthe power-tube of the amplifier, and this tube requires a larger plate current than the preceding tubes, or a current which dominates the plate current in the other tubes.

It will be seen that the impedance of the rectifier tube and the smoothing filter in thecire cuit in Figure 1 is included in each of the plate circuits. ofthe various st g aa d consequently, current variations in theplate circuit of the power tube will produce voltage variations on ,the vother tubes of the preceding stages. This gives rise to low frequency oscillations commonly known as motor-boating which are produced by the feeding back of energy from the power stage to a preceding. stage through the mutual impedance of the plate supply source. This effect is particularly troublesome in resistancecoupled amplifiers, and is present in a lesser degree of mitigatmgthe difficulty isto provide the individual plate circuit filters referred v to above. These filters are bulky and expensive,

and in the caseof resistance coupled amplifiers, are frequently inadequate to prevent the low frequency disturbances.

In the arrangement .hown in Figure 2 in which my invention is embodied, the plate current supply source, including the alter-' hating current sourceo, rectifier 6, andthe filter 7, forms one arm D of a Wheatstone bridge, theremainingarms of whichare indicated at A, Band 0. Plate current to the power tube 4 is supplied over a path connectedbetvveen the diagonal points E audF of the bridge, and plate current for relays 1 to 3 is suppliedover a path connected betvveenthe diagonal points G and H, of the bridge. When the bridgeis properly balsanced, these two diagonal circuit paths are N that supplied to the power tube. I

Shouldan intermediate tube or tubes produce objectionable, feed-back into a preceding.stage, a second bridge may be provided to supply theplate current for'such tube or of the elements comprising the arms G will depend somewhat upon the resistance of the current supply source. It will beconvenient to makethe value of the resistance in arm C such that the resistance drop occasioned by the flow-of current through this resistance will provide the required grid biasing voltage for the power tube 4:. The proportion betweenresistances of arms C and D then establishes the ratio for the bridge arms A and B. As an example, the constants suitable for a common type of power tube may be stated:

This tube requires a plate voltage of 100 volts, agrid voltage of 70 volts, and draws a plate current of 55 milliamperes. A resistanceof approximately 1200 ohms in arm" C will then supply the 70 volts vnecessary for biasing'the grid of thetuber A common value of resistance found 1n the combined current supply elements 5, 6 and 7 is about "600 ohms.

Thus the resistances in armA and Bshould be in the ratio. of approximately I to .2. Values of 5000 and 10,000 ohms respectively have been foundsatisfactory. The voltage 1 supplied to tube 3 will then be two-thirds of tubes. Forexample, as shown in Fig. 2, the plate current for tube2 and tube 3 is supplied over circuit paths connected between diagonal points E and F of a second bridge AB'-C D, and the plate current for relay 1 is supplied over a'path connected between diagona-lpoints H and Gt of the sec- 0nd bridge. In this case it is only'necessary not generally necessary.

From Figure 2, it willbe seen that two' that the arm C balance the combined resistance of the current supply: network up to i the second bridge. When thesecond bridge is balanced in the manner describedabove for the first bridge, plate current variations in tubes 2 and 3 will-not react upon tube 1. Due to the type oftube usually employed in intermediatestages this second bridge will Wheatstone, bridges are provided, and one .arm (arm D) of one bridge is included in a diagonalpath of the other bridge, and the plate current supply of power tube l is over the second diagonal path of bridge AB C-D. -The plate current supply. for tubes 2 and 3 is over a diagonal path of bridge A-B- and plate current supply I V w saidrelays, a 'Wheatstone bridge network includingfsaidsourcein onelofthe arms, one of said relays being connected in one diagonal for tube 1 is over the second diagonal path for bridge A, B, C. u

The arrangement above described has been found successful in curing persistent low frequency oscillation in resistance coupled amplifiers. The nature of the balance is not critical, which is a useful feature, inasmuch as the resistance of the rectifier tube varies somewhat in accordance with the voltage of the main power supply. The impedance of the combined power supply elements is primarily resistive, but series inductance and shunt-capacity maybe added if necessary to secure a satisfactory balance. As a rule, the ratio of the resistances in arms D and C will be greater than one so that the voltage available for the preceding tubes will be somewhat larger than one-half the value of the 'maximum rectified voltage. This method of feed-back neutralization is applicable also where plate current is supplied by a relatively low resistance battery. It may bedesirable in that case to add series resistance in arm 5 D to secure the desired voltage proportions.

While the invention is illustrated as embodied in a particular type of amplifier which is particularly susceptible to low frequency oscillations, it is'a-pplicable generally to any groupof dev ces having a common sourceof potential possessin'gimpedance.

I claim: V v 1. In combination, a plurality of current an alternating current circuit, a common source of direct current for energizing said devices, and circuits connecting said devices to said source in circuit eachother. e

2. In combination, a plurality ofelectric relays,"a common source of direct current for said relays, and circuits connecting said relays to sald source in circuit paths con ugate to each other." e c 3. In combination, a plurality of vacuum th t t pa s con uga e 0 ming devices connected in cascade in tube relays,a common source ofspace eurrent for said relays having sensible impedance, one of said relays having a dominant plate current, and neutralizing means for preventing current variations of said relay from reacting on the other relays through the space current source. f b

' 4. {In combination, a plurality of vacuum tube relays, a common source of space current for said'relays, and circuits connecting said relays :to said source in. circuit paths cQn ugate to eachother.

5. Incombination, a plurality of vacuum tube relays, a common source of space current for said relays, and means for connectingthe anode-cathode elements of said relays .tosaid source in circuit paths conjugate to each other. i a e 6.- II1 COII1biI 1atlOI1, a pair of vacuum. tube relays, a common source of space current for path of said bridge and the other relay in the second diagonal path.

7. In a signaling system, a plurality of vacuum tube relays connected in cascade for amplifyingvariable currents, a common source of space current for said relays, and space current supply circuits connecting said relays to said source in circuit paths conjugate to each other.

19 8. In a signaling system, a plurality of vacuum tube relays connected in cascade for amplifying variable currents, a common source of space current for said relay, a circuit network connecting said source and relays including a Wheatstone bridge, said source being included in one arm of said bridge, and adjacent tubes of said cascade being included in the diagonal paths respectively of said bridge.

20 9. In combination, a pair of vacuum tube relays, a common source of space current for said relays, a circuit for said relays, a circuit network comprising a \Vheatstone bridge one arm of which includes said source and one diagonal path includes one of said relays, a second Wheatstone bridge one arm of which in cludes the second diagonal path of said first bridge, said second relay being included in a diagonal path of said second bridge.

10. In a signaling system, a plurality of vacuum tube relays connected in cascade for amplifying variable currents, a common source of space current for said relays, space current circuits connecting said source and said relays including a plurality of Wheat stone bridge networks each including one of said relays in one of its diagonal paths, and each bridge except the last including in one of its arms a diagonal path of another bridge.

11. In a signaling system, a plurality of vacuum tube relays connected in cascade for amplifying variable currents, a common source of space current for said relays, space current circuits connecting said source and said relays including a plurality of Wheatstone bridge networks each including one of said relays in one of its diagonal paths, and each bridge except the last including in one of its arms a diagonal path of another bridge,

said source of space current being included in an arm of the last bridge in the cascade.

In testimony whereof I affix my signature.

JOHN R. HYNEMAN. 

